| Prior treatment for the intercritical quenching of cast steel ZG25MnCrNiMo was established based on the conventional heat treatment, and the morphology, distribution and quantity of the undissolved ferrite in the intercritically quenched cast steel subjected to different prior treatment were investigated. The Charpy impact energy of the intercritically treated cast steel was compared with that treated by conventional quenching and tempering.Experiments showed that the amount of undissolved ferrite in ZG25MnCrNiMo is extremely rare when heated to 790℃ or above. The net form of original casting structure can not be removed by conventional heat treatment or intercritical quenching process. The microstructure obtained by lower-temperature intercritical quenching is more uniform than that obtained by higher-temperature intercritical quenching. The dispersity of ferrite-matensite in the intercritically quenched microstructure subjected to a quenching and tempering prior heat treatment is better than that in the intercritically quenched microstructure subjected to a quenching prior heat treatment.The form of undissolved ferrite obtained by intercritical quenching process could be long needle strips, short bundles, small particles and irregular polygonal shapes, and the form of martensite could be bundles in two-phase area, nets along original austenite grain boundaries consisting of individual martensite particles, and lumps.By proper intercritical treatment, the Charpy impact energy of cast steel ZG25MnCrNiMo under -40℃ can be 10% higher than that obtained by conventional quenching and tempering possessing a similar strength level. |
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